This invention relates to a sensor for measuring the concentration of a solute or a solvent in a mixed solution system, and in particular to a sensor for measuring the concentration of a low molecular weight alcohol, such as methanol and ethanol, particularly in an aqueous liquid feed solution for a fuel cell, e.g., a direct methanol fuel cell (DMFC), a direct ethanol fuel cell (DEFC), DMFC/DEFC test stations, and in aqueous solutions at ethanol/methanol refineries, alcohol production plants, chemical labs, and other systems where a solvent or solute concentration needs to be determined and monitored.
Liquid feed fuel cells operate directly on an organic liquid fuel stream, typically supplied as an aqueous fuel solution. Ethanol is used as the fuel, although methanol is predominantly used.
Direct methanol fuel cells/ethanol fuel cells have been the subject of considerable research during the last ten years. As a result, significant improvements in power density, efficiency, and lifetime have been reported. Performance levels achieved in cells, stacks, and systems show that this technology is a promising power source for a wide range of portable applications.
A direct methanol fuel cell (DMFC) is a type of solid polymer fuel cell that operates directly on a methanol fuel stream typically supplied as a methanol/water vapour or as an aqueous methanol solution in liquid feed DMFCs. The methanol in the fuel stream is directly oxidized at the anode therein. There is often a problem in DMFCs with crossover of methanol fuel from the anode to the cathode side through the membrane electrolyte. The methanol that crosses over typically then reacts with oxidant at the cathode and cannot be recovered, resulting in significant fuel inefficiency and deterioration in fuel cell performance. To reduce crossover, dilute solutions of methanol (for example, about 5 wt % methanol in water) are typically used as fuel streams in liquid feed DMFCs. The fuel streams in DMFCs are usually recirculated in order to remove carbon dioxide (a by-product of the reaction at the anode) and to re-use the diluent and any unreacted fuel in the depleted fuel stream exiting the DMFC. Methanol is added to the circulating fuel stream before it re-enters the fuel cell in order to compensate for the amount consumed, thereby providing a fresh mixture at the desired methanol concentration. Since the amount of methanol consumed is variable (depending on the load, crossover, and other operating parameters), the methanol concentration in the circulating fuel stream is usually measured continuously with a suitable sensor, and fresh methanol is admitted in accordance with the signal from the sensor. The concentration of methanol in the fuel circulation loop is an important operating parameter because it determines the electrical performance and efficiency of the direct methanol fuel cell system. The practical operation of direct methanol fuel cell systems requires accurate monitoring and control of methanol concentration, which is strongly dependent on a methanol concentration sensor.
A viable methanol concentration sensor should have a sensitivity of about ±0.02 M over the range of 0.1-2 M, with a response time of less than 1 s, although in general such specifications would be highly dependent on the mode of operation and the application. There are many factors to consider in developing a methanol sensor suitable for DMFCs. These factors include cost, size, simplicity, reliability, longevity, concentration range, and dynamic response. In particular, reliability and low cost should be addressed.
Methanol concentration sensors measure methanol concentration by means of detecting the variations of physical/chemical properties of the solution. Various types of sensors have been considered for the purposes of measuring the concentration of methanol in aqueous solution and thus for use in a recirculating fuel stream in a liquid feed DMFC.
1). Electrochemical-Based Sensors
An electrochemical-based sensor is a small DMFC, generally based on the electro-oxidation of methanol to carbon dioxide on platinum ruthenium catalysts [Barton, S. A. C., Muranch, B. L., Fuller, T. F., and West A. C., J. Electrochem. Soc. 1998, 145, 3783-3788; S. R. Narayanan, S. R., Valdez, T. I., and Chun, W., “Design and operation of an electrochemical methanol concentration sensor for direct methanol fuel cell,”, Electrochemical and Solid-State Letters 2000, 3, 117-120; Qi, Z., He, C., Hollett, M., Attia, A., and Kaufman, A., “Reliable and fast-responding methanol concentration sensor with novel design,” Electrochemical and Solid-State Letters 2003, 6, A88-A90; H. Zhao, J. Shen, J. Zhang, H. Wang, D. P. Wilkinson, C. E. Gu, Journal of Power Sources 2006, 159, 626-636].
By keeping the anode potential at a certain voltage (>0 V and ≦0.7V), the current produced by the sensor is proportional to the concentration of the methanol solution. Thus this type of sensor electrochemically measures the concentration of methanol aqueous solution used in a DMFC system.
This type of sensor is probably the most popular one.
2). Electric-Capacitance Type Sensors
This type of sensor measures the change in dielectric constant of the fuel stream. The capacitance of a capacitor is measured with the methanol aqueous solution placed as a dielectric. The dielectric constant of the solution is proportional to the measured capacitance, and upon determining the dielectric constant of the solution, the methanol concentration in the mixture or in the electrolyte can be calculated [e.g. see US 2002/109511].
3). Infrared Sensing
This sensing technique uses an infrared sensor device for measuring methanol's content in water solutions in the fuel circulation loop of a DMFC [e.g. see U.S. Pat. No. 6,815,682]. The infrared sensor is designed based on a specific methanol absorption peak in the infrared range. The IR spectrum of an aqueous solution of methanol shows a well-distinguished absorption peak at a wavelength of about 9.85 micrometers. The amplitude of the absorption peak is proportional to methanol concentration.
4). Ultrasound Sensing
The speed of sound in a water-methanol system increases significantly with methanol content; techniques for measuring characteristic sound velocities are used to measure methanol concentration [e.g. see U.S. Pat. No. 6,748,793].
5). Other Techniques
Other techniques include the measurement of viscosity or heat capacity or boiling temperature of methanol solution to determine the methanol concentration. There is one technique eliminating the need for a separate sensor by calculating the methanol concentration as a function of the observed current, the temperature of the fuel stream entering the DMFC stack, and the temperature of the DMFC stack itself [e.g. see U.S. Pat. No. 6,698,278]. Coulometric methods are really only accurate when there are no other H3OH losses (such as cross-over, etc.).
Disadvantages and Limitations of the Prior Art Sensors
1). Electrochemical-based sensors suffer from degradation of the electrode reaction resulting in performance deterioration or failure over time.
2). Electric-capacitance type sensors measure the change in dielectric constant of the fuel stream. In theory, the larger the difference between the dielectric properties of two components of the fuel stream, the more precise the measurement can be. Unfortunately, the difference in dielectric constants for methanol-water systems is relatively small which may lead to misleading results or failure. For example, capacitance measurement has been used as a means of monitoring methanol concentration in a mixture of gasoline and methanol [e.g. see U.S. Pat. Nos. 4,939,467 and 5,196,801]. Due to the difference in dielectric constants between methanol and gasoline, the capacitance between two electrodes changes with the methanol concentration. Unfortunately, because the dielectric constant difference between water and methanol is much less than that between gasoline and methanol, and the methanol concentration used in a DMFC is normally less than 5 wt %, such a method can hardly provide a satisfactory measure of methanol concentration in water.3). The infrared sensing has been proven effective for measuring the concentration of methanol in the range 0% to 5 wt % in water solution. For a wider range, this technique needs to be proven.4). The ultrasound sensing system is not easy to miniaturized, and the cost is not low.